This invention relates generally to 2-phenylpyridine derivatives, and more particularly, to ferroelectric liquid crystal materials including the 2-phenylpyridine derivatives that are useful in ferroelectric liquid crystal display devices.
Ferroelectric liquid crystal materials have recently attracted attention due to their high response speed and good memory properties as discussed in N. A. Clark et al, Applied Physics Letters, Vol. 36, p. 899 (1980). Similar properties are not obtainable using prior art liquid crystal materials. In recent years investigators have sought practical uses for such ferroelectric liquid crystal materials. Accordingly, there is a need to develop ferroelectric liquid crystal materials that are suitable for practical use.
A "ferroelectric liquid crystal material" as used herein is a liquid crystal material which exhibits ferroelectric properties as follows. When an electric field is applied to a dielectric liquid crystal having permanent dipoles, the dipoles are oriented with the direction of the applied electric field and dielectric polarization occurs. When electrostatic interaction between permanent dipoles is strong, the dipoles are arranged in parallel even without application of the external electric field. When dipoles are arranged in parallel, spontaneous polarization exists. The property that the direction of this spontaneous polarization can be reversed by externally applying an electric field is known as "ferroelectricity", and a liquid crystal material having this property is a "ferroelectric liquid crystal material".
In general, liquid crystal materials having an optically active portion exhibit ferroelectric properties in smectic phases. The molecular arrangement is such that the major axis of the molecules is tilted from the normal axis of the smectic liquid crystal layers. It is highly desirable that the liquid crystal material exhibit a chiral smectic C phase ("SmC*") for practical operation of the liquid crystal material over a relatively low voltage range.
Okano and Kobayashi et al introduced compounds of Schiff series, azoxy series and ester series in their publication entitled "The Liquid Crystal--The Basic Edition", p. 140 (1985) as typical ferroelectric liquid crystal materials. However, the Schiff series, azoxy series and ester series compounds have significant disadvantages including chemical instability, low light resistance, high viscosity and the like. Accordingly, at the present time there are a limited number of ferroelectric liquid crystal materials that are suitable for practical use.
When a ferroelectric liquid crystal material is to be used as the electro-optical element in a liquid crystal display cell, some of the properties including temperature range and other electro-optical properties can be adjusted by mixing the material with other types of ferroelectric liquid crystal materials or with non-ferroelectric liquid crystal materials having the desired properties. However, such techniques are not always entirely suitable.
Accordingly, it is desirable to provide an improved ferroelectric liquid crystal material.